Polymerization catalyst system using n-butymethyldimethoxysilane for preparation of polypropylene film grade resins

ABSTRACT

It has been discovered that using n-butylmethyldimethoxysilane (BMDS) as an external electron donor for Ziegler-Natta catalysts can provide a catalyst system that may prepare polypropylene films with improved properties. The catalyst systems of the invention provide for controlled chain defects/defect distribution and thus a regulated microtacticity. Consequently, the curve of storage modulus (G′) v. temperature is shifted such that the film achieves the same storage modulus at a lower temperature enabling faster throughput of polypropylene film through a high-speed tenter.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is a divisional of co-pending application Ser.No. 10/233,637 filed Sep. 3, 2002, by Kenneth Paul Blackmon, David J.Rauscher and Michael Ray Wallace, under the same title.

FIELD OF THE INVENTION

[0002] The present invention relates to polymerization catalyst systemsand processes for the preparation of polypropylene, and moreparticularly relates, in one embodiment, to polymerization catalystsystems for and controlled polymerization processes for the preparationof polypropylene of specified microtacticity that give improvement inphysical properties and processability of polypropylene film.

BACKGROUND OF THE INVENTION

[0003] Thermoplastic olefin polymers, such as linear polyethylene,polypropylene, and olefin copolymers, are formed in polymerizationreactions where a monomer is introduced into a reactor with anappropriate catalyst to produce the olefin homopolymer or copolymer. Thepolymer is withdrawn from the catalyst reactor and may be subjected toappropriate processing steps and then extruded as a thermoplastic massthrough an extruder and die mechanism to produce the polymer as a rawmaterial in particulate form, usually as pellets or granules. Thepolymer particles are ultimately heated and processed in the formationof the desired end products.

[0004] Polypropylene manufacturing processes typically involve thepolymerization of propylene monomer with an organometallic catalyst ofthe Ziegler-Natta type. The Ziegler-Natta type catalyst polymerizes thepropylene monomer to produce predominantly solid crystallinepolypropylene. Polypropylene is most often produced as a stereospecificpolymer. Many desirable product properties, such as strength anddurability, depend on the crystallinity of the polypropylene that inturn is dependent on the stereospecific arrangement of methyl groups onthe polymer backbone.

[0005] Stereospecific polymers are polymers that have a definedarrangement of molecules in space. Both isotactic and syndiotacticpropylene polymers, for example, are stereospecific. The isotacticstructure is typically described as having the methyl groups attached tothe tertiary carbon atoms of successive monomeric units on the same sideof a hypothetical plane through the main chain of the polymer, e.g., themethyl groups are all above or all below the plane. Isotacticpolypropylene can be illustrated by the following chemical formula:

[0006] This structure provides a highly crystalline polymer molecule.Using the Fisher projection formula, the stereochemical sequence ofisotactic polypropylene may be shown as follows:

[0007] Another way of describing the structure is through the use of NMRspectroscopy. Bovey's NMR nomenclature for an isotactic pentad is mmmmwith each “m” representing a “meso” dyad or successive methyl groups onthe same side in the plane. As known in the art, any deviation orinversion in the structure of the chain lowers the degree ofisotacticity and crystallinity of the polymer.

[0008] This crystallinity distinguishes isotactic polymers from anamorphous or atactic polymer, which is soluble in an aromatic solventsuch as xylene. Atactic polymer exhibits no regular order of repeatingunit configurations in the polymer chain and forms essentially a waxyproduct. That is, the methyl groups in atactic polypropylene arerandomly positioned. While it is possible for a catalyst to produce bothamorphous and crystalline fractions, it is generally desirable for acatalyst to produce predominantly crystalline polymer with very littleatactic polymer.

[0009] Catalyst systems for the polymerization of olefins are well knownin the art. Typically, these systems include a Ziegler-Natta typepolymerization catalyst; a co-catalyst, usually an organoaluminumcompound; and an external electron donor compound or selectivity controlagent, usually an organosilicon compound. Examples of such catalystsystems are shown in the following U.S. Pat. Nos.: 4,107,413; 4,294,721;4,439,540; 4,115,319; 4,220,554; 4,460,701; and 4,562,173; thedisclosures of these patents are hereby incorporated by reference. Theseare just a few of the scores of issued patents relating to catalysts andcatalyst systems designed primarily for the polymerization of propyleneand ethylene.

[0010] Ziegler-Natta catalysts for the polymerization of isotacticpolyolefins are well known in the art. The Ziegler-Natta catalysts arestereospecific complexes derived from a halide of a transition metal,such as titanium, chromium or vanadium with a metal hydride and/or metalalkyl, typically an organoaluminum compound as a co-catalyst. Thecatalyst is usually comprised of a titanium halide supported on amagnesium compound. Ziegler-Natta catalysts, such as titaniumtetrachloride (TiCl₄) supported on an active magnesium dihalide, such asmagnesium dichloride or magnesium dibromide, as disclosed, for example,in U.S. Pat. Nos. 4,298,718 and 4,544,717, both to Mayr, et al. aresupported catalysts. Silica may also be used as a support. The supportedcatalyst may be employed in conjunction with a co-catalyst such as analkylaluminum compound, for example, triethyl aluminum (TEAL), trimethylaluminum (TMA) and triisobutyl aluminum (TIBAL).

[0011] The development of these polymerization catalysts has advanced ingenerations of catalysts. The catalysts disclosed in the patentsreferenced above are considered by most to be third or fourth generationcatalysts. With each new generation of catalysts, the catalystproperties have improved, particularly the efficiencies of thecatalysts, as expressed in kilograms of polymer product per gram ofcatalyst over a particular time.

[0012] In the utilization of a Ziegler-Natta catalyst for thepolymerization of propylene, it is generally desirable to add anexternal donor. External donors act as stereoselective control agents tocontrol the amount of atactic or non-stereoregular polymer producedduring the reaction, thus reducing the amount of xylene solubles.Examples of external donors include the organosilicon compounds such ascyclohexylmethyldimethoxysilane (CMDS), dicyclopentyldimethoxysilane(CPDS) and diisopropyldimethoxysilane (DIDS). External donors, however,tend to reduce catalyst activity and tend to reduce the melt flow of theresulting polymer.

[0013] In addition to the improved catalysts, improved activationmethods have also lead to increases in the catalyst efficiency. Forexample, one discovery involved a process for pre-polymerizing thecatalyst just prior to introducing the catalyst into the reaction zone.

[0014] It is generally possible to control catalyst productivity (i.e.,lbs. of polypropylene/lb. catalyst or other weight ratios) and productisotacticity within limits by adjusting the molar feed ratio ofco-catalyst to external electron donor (and their corresponding ratiosto the active metal content, e.g., titanium, in the Ziegler-Nattacatalyst). Increasing the amount of external electron donor decreasesthe xylene solubles but may reduce activity and hence catalystproductivity. The xylene solubles (XS) content of the polypropyleneproduct is a measure of the degree of stereoselectivity. Further, thepolymer stereoregularity may be obtained by directly measuring themicrotacticity of the product via ¹³C Nuclear Magnetic Resonancespectroscopy.

[0015] Selectivity to isotactic polypropylene is typically determinedunder the XS test by measuring the amount of polypropylene materialsthat are xylene soluble. The xylene-solubles were measured by dissolvingpolymer in hot xylene, cooling the solution to 0° C. and precipitatingout the crystalline material. The xylene solubles are the wt. % of thepolymer that was soluble in the cold xylene.

[0016] In particular with respect to film grade polyolefin resins forbiaxially oriented polypropylene (BOPP) applications, there iscontinuing interest in identifying catalyst systems that offer potentialimprovements in polymer physical properties and processability. Someprevious studies have focused on efforts to enhance resinprocessability/extrusion characteristics via broadening of polymermolecular weight distribution through utilization of particular donortypes (e.g., bis(perhydroisoquinolino)dimethoxysilane (BPIQ)). Other,more recent studies have focused on the use of fluoroalkylsilanecompounds (e.g., 3,3,3-trifluoro-propylmethyldimethoxysilane (“E”donor)) that potentially allow for a controlled lower polymerstereoregularity and slightly lower polymer melting temperature, therebypotentially improving resin processability during film production.Indeed, these various catalyst system approaches to the modification ofpolymer properties for potential enhancement of film gradecharacteristics have shown varying degrees of promise.

[0017] It would be particularly advantageous to determine the optimumtype of external donor and molar ratio of co-catalyst to externalelectron donor in order to obtain a desirable polymer stereoregularityand minimize the amount of xylene solubles in polypropylene.

SUMMARY OF THE INVENTION

[0018] Accordingly, it is an object of the present invention to providea catalyst, a method of making a catalyst, and a method of using thecatalyst for polymerization or copolymerization of propylene to producea polypropylene product having a controlled amount of xylene solubles.

[0019] It is another object of the present invention to provide acatalyst, a method of making a catalyst, and a method of using thecatalyst for polymerization or copolymerization of propylene to producea polypropylene product having a controlled microtacticity.

[0020] Still another object of the invention is to provide a catalyst, amethod of making a catalyst, and a method of using the catalyst forpolymerization or copolymerization of propylene to produce apolypropylene product having these controlled properties using anelectron donor that is relatively inexpensive.

[0021] In carrying out these and other objects of the invention, thereis provided, in one form, a catalyst system for the polymerization orcopolymerization of propylene monomer having a Ziegler-Natta catalyst,an organoaluminum compound co-catalyst, and at least one externalelectron donor comprising n-butylmethyldimethoxysilane (BMDS).

[0022] In another embodiment of the invention, there is provided aprocess for the polymerization or copolymerization of propylene monomerthat involves (a) providing a Ziegler-Natta catalyst, (b) contacting thecatalyst with an organoaluminum compound, (c) contacting the catalystwith at least one electron donor comprising n-butylmethyldimethoxysilane(BMDS) simultaneously with or subsequent to (b), (d) introducing thecatalyst into a polymerization reaction zone containing theorganoaluminum compound, the electron donor and propylene monomer; and(e) removing polypropylene homopolymer or copolymer from thepolymerization reaction zone.

[0023] In yet another embodiment of the invention, there is providedpolypropylene that encompasses a propylene polymer or copolymer having amelt flow of at least about 0.5 g/10 min. and xylene solubles of notmore than about 6 weight %, and a meso pentad level of between about 91and about 98%, as measured by ¹³C NMR on the insoluble fraction (i.e.,that portion which is insoluble in xylene and, subsequently, heptane).In still another embodiment of the invention, the invention concernsarticles made from the polypropylene of this invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 is a graph of Storage Modulus (G′) v. Temperature forlaboratory “film grade” resins prepared with a conventional Ti on MgZiegler-Natta catalyst and various donors (polymer characterization viaa Rheometrics Dynamic Analyzer RDA II using a rectangular torsiontemperature sweep; at a frequency of 300 rad/s and 1% strain); and

[0025]FIG. 2 is a graph of Yield Stress v. Temperature for compressionmolded film grade resins on a Bruckner film stretcher using a stretchvelocity of 3 m/min. and a stretch ratio of 4.

DETAILED DESCRIPTION OF THE INVENTION

[0026] It has been surprisingly discovered that the use of aZiegler-Natta catalyst that includes n-butylmethyldimethoxysilane (BMDS)as an external electron donor for the polymerization of propylene canyield a polymer with a controlled microtacticity. More specifically, newpolypropylene with tailored and controlled chain defects/defectdistribution can result in improved stretchability of polypropylenefilms. These films may attain the same storage modulus (G′, i.e.stiffness) at a lower temperature as otherwise comparable polypropylenefilms. As a result, the films may be heated faster to give very similarproperties as compared with films from other polypropylene, andthroughputs in high-speed film processing equipment can be increased. Ina non-limiting instance, polypropylene film is desired to be produced atrates of about at least 300 and up to 500 m/min. (meters/minute).

[0027] The Ziegler-Natta catalysts useful in the present inventioninclude those derived from a halide of a transition metal, such astitanium, chromium or vanadium, with titanium being the preferred metalin many embodiments. Examples of transition metal compounds include, butare not necessarily limited to, TiCl4, TiBr₄, TiO(C₂H₅)₃Cl,Ti(OC₂H₅)₃Cl, Ti(OC₃H₇)₂Cl₂, TiO(C₆H₁₃)₂Cl₂, Ti(OC₂H₅)₂Br₂ andTi(OC₁₂H₂₅)Cl₃. The tra used individually or in combination. Typicaltitanium levels are from about 1.0% to about 5.0% by weight of catalyst,in one non-limiting embodiment of the invention. The Ziegler-Nattacatalyst may be a transition metal compound of the formula MR⁺ _(x)where M is selected from the group consisting of titanium, chromium, andvanadium, R is selected from the group consisting of halogen or ahydrocarboxyl, and x is the valence of M.

[0028] The transition metal halide is used in combination with a metalhydride and/or metal alkyl, typically an organoaluminum compound as aco-catalyst. Desirably the co-catalyst is an aluminum alkyl having theformula AIR₃, where R is an alkyl group having 1 to 8 carbon atoms, withR being the same or different. Examples of suitable aluminum alkylsinclude, but are not necessarily limited to, trimethyl aluminum (TMA),triethyl aluminum (TEAL) and triisobutyl aluminum (TIBAL). In onenon-limiting embodiment of the invention, the desired aluminum alkyl isTEAL.

[0029] In one non-limiting theory about the mechanism by which theinvention herein functions, the external donor operates by counteringthe loss of internal donor in the catalyst system. The nature of theinternal donor is not particularly critical to the catalyst and itsmethod of use in this invention, as long as the goals and objectives ofthe invention with respect to the polypropylene product are met.Suitable internal donors include, but are not necessarily limited to,diethers (such as those discussed in U.S. Pat. Nos. 4,971,937 and5,106,807, which are incorporated herein by reference), aromaticdiesters such as alkyl phthalate donors (e.g. diethyl phthalate,di-isobutyl phthalate, such as those listed in U.S. Pat. No. 5,945,366,which is also incorporated herein by reference), amines, amides,ketones, nitriles, phosphines, thioethers, thioesters, aldehydes,alcoholates, salts of organic acids, and combinations thereof. Preferredinternal donors include, but are not necessarily limited to, esters ofphthalic acid such as di-iso-butyl, dioctyl, diphenyl, and benzylbutyl,and the like, and combinations thereof.

[0030] These internal electron donors are added during the preparationof the catalysts and may be combined with the support or otherwisecomplexed with the transition metal halide.

[0031] The Ziegler-Natta catalyst is typically a supported catalyst.Suitable support materials include magnesium compounds, such asmagnesium halides, dialkoxymagnesiums, alkoxymagnesium halides,magnesium oxyhalides, dialkylmagnesiums, magnesium oxide, magnesiumhydroxide, and carboxylates of magnesium. Typical magnesium levels arefrom about 12% to about 20% by weight of catalyst.

[0032] In the subject invention, the Ziegler-Natta catalyst must be usedwith at least one external donor compound, such as a Lewis base. Morespecifically, external donors are typically organosilicon compounds.External electron donors may be those described by the formulaSiR_(m)(OR′)_(4-m), where R is an alkyl group, a cycloalkyl group, anaryl group or a vinyl group, R′ is an alkyl group, m is 0-4, each R′ maybe the same or different, and each R may be the same or different. Inparticular, the external electron donor acts as a stereoregulator tocontrol the amount of atactic form of polymer produced, which results isin a decrease in xylene solubles. Examples of electron donors that areorganic silicon compounds are disclosed in U.S. Pat. Nos. 4,218,339;4,395,360; 4,328,122; 4,473,660 and 4,927,797, which are incorporatedherein by reference. Representative examples of external donors includecyclohexylmethyldimethoxysilane (CMDS), dicyclopentyldimethoxysilane(CPDS), diisopropyldimethoxysilane (DIDS),cyclohexylisopropyldimethoxysilane (ClDS), di-t-butyldimethoxysilane(DTDS), (3,3,3-trifluoropropyl)methyldimethoxysilane (“E” donor), andcombinations thereof. However, in the subject invention, at least one ofthe electron donors that should be used is n-butylmethyldimethoxysilane(BMDS). As discussed, BMDS has been discovered to be used withZiegler-Natta catalysts to produce polypropylene with a lower degree ofmicrotacticity that is advantageous for BOPP film processability, butwhile retaining desirable melt flow and xylene solubles levels. It iswithin the scope of this invention to use BMDS in conjunction with oneor more other external donors including, but not necessarily limited to,CMDS, CPDS, DIDS, ClDS, DTDS and/or “E” donor. In some cases it will befound that there is a synergistic effect between the internal donor andthe external donor. That is, results will be obtained with a particularcombination of internal donor and external donor that cannot be achievedwith one or the other individually.

[0033] Unless specified otherwise, amounts of external donor arepresented herein as parts per million (ppm) based on the weight ofmonomer. In one non-limiting embodiment of the invention, the amount ofBMDS ranges from about 0.5 to about 500 ppm, preferably from about 2 toabout 200 ppm, and most preferably from about 4 to about 20 ppm.Desirably, any second or subsequent external donor is used in the rangeof from about zero to about 5 ppm, with from about zero to about 3 ppmbeing preferred, from about zero to about 2 ppm being more preferred,from about zero to about 1.5 ppm being even more preferred, from aboutzero to about 1 ppm being still more preferred, and from about zero toabout 0.5 ppm being still more preferred. The Al/Si molar ratio(organoaluminum compound to silane donor) may range from about 0.5 toabout 500, preferably from about I to about 200, and most preferablyfrom about 1 to about 100.

[0034] As is well known, polypropylene may be produced by slurrypolymerization in the presence of a solvent, e.g. hexane, such as in aloop or CSTR reactor, or by bulk polymerization in which propyleneserves as both monomer and diluent, which is typically carried out in aloop-type reactor. Also, polypropylene may be produced by gas phasepolymerization of propylene, which is typically carried out in afluidized bed reactor under lower pressures than bulk polymerization. Ina typical bulk process, one or more loop reactors operating generallyfrom about 50 to about 100° C. (preferably from about 60 to about 80°C.), with pressures of from about 300 to 700 psi (2.1 to 4.8 MPa)(preferably from about 450 to about 650 psi) (3.1 to 4.5 MPa), may beused to polymerize propylene. The various catalytic components, i.e.,Ziegler-Natta catalyst, cocatalyst, external donor, are introduced intothe reactor, as well as a molecular weight controlling agent (if any,e.g., hydrogen), and the resulting polypropylene fluff or powder iscontinuously removed from the reactor. The fluff may then be subjectedto extrusion to produce desired pellets.

[0035] In the study of this invention, a conventional titanium supportedon an active magnesium dihalide Ziegler-Natta catalyst was used in thepresence of three external silane donors to assess effects onpolymerization performance and polymer properties. As will be discussed,it was discovered that BMDS yields a desirable decrease in polymermicrotacticity, on the same order as “E” donor. Advantageously, BMDS isconsiderably less expensive than “E” donor.

[0036] For bulk polymerization utilizing the BMDS externaldonor-containing catalyst, the reactor temperatures are usually keptfrom about 50 to about 100° C., more particularly from about 60° C. toabout 80° C. It should be noted that increasing the temperature (withinlimits) will typically result in an increased catalytic activity andlower xylene solubles. Hydrogen concentrations may vary, but are usuallykept at from about 0.02 mol % to about 1.1 mol %, more particularly fromabout 0.04 mol % to about 0.5 mol % based on monomer, and depending onthe resin melt flow desired.

[0037] The polymers produced in accordance with the present inventionare those having a melt flow after polymerization of at least 0.5 g/10min or greater, as measured according to ASTM D1238-95. Typical meltflows useful for preparation of BOPP film are from about 0.5 g/10 min toabout 20 g/10 min, with from about 1.0 g/10 min to about 10 g/10 minbeing readily obtainable. Melt flows above 30 g/10 min, 50 g/10 min andeven above 100 g/10 min are attainable, while still retaining low xylenesolubles. Thus, the polymers of this invention are expected to besuitable for film grade resins as well as for injection moldingapplications, and the like. The polymers produced are also characterizedas having low xylene solubles of not more than about 6 weight %, withfrom about 1 to about 5% being readily obtainable, and from 2 to about4% being more readily obtainable, without any detrimental effects onmelt flow. Additionally, the polypropylene homopolymer or copolymer mayhave a meso pentad level of between about 91 to about 98%, preferablyfrom about 91-95% and most preferably from about 92 to about 95% asmeasured via ¹³C NMR on the insoluble (i.e., crystalline) fraction. Thepolydispersity (Mw/Mn) of the polypropylene homopolymer or copolymer, asmeasured via Size Exclusion Chromatography, may range from about 4 toabout 10, preferably from about 6 to about 9.

[0038] As used herein, the terms “propylene polymer” or “polypropylene,”unless specified otherwise, shall mean propylene homopolymers or thosepolymers composed primarily of propylene and limited amounts of othercomonomers, such as ethylene, wherein the comonomers make up less than0.5% by weight of polymer, and more typically less than 0.1% by weightof polymer. However, in some cases, minirandom copolymers with evensmall amounts of ethylene are not desired. The catalysts of thisinvention provide another way of adjusting the microtacticity of thepolypropylene and thus improving the properties of film gradepolypropylene.

[0039] The following examples serve to illustrate the present invention,but are not intended to limit the invention in any way.

[0040] A conventional titanium supported on an active magnesium dihalideZiegler-Natta catalyst was used in the presence of three silane externaldonors to assess effects on polymerization performance and polymerproperties. This conventional Ziegler-Natta catalyst typically containsapproximately 2.8% titanium by weight and approximately 19.2% magnesiumby weight, with an average particle size in the range of 10 to 14microns, and is designated herein as Catalyst X. These donors includedcyclohexylmethyldimethoxysilane (CMDS, “C” donor),3,3,3-trifluoropropylmethyldimethoxysilane (“E” donor), andn-butylmethyldimethoxysilane (BMDS). Of particular interest was theeffect on catalyst activity, polymer microtacticity, and polymer dynamicmechanical behavior. The general experimental conditions and reagentsfor the catalyst evaluations are shown in Table I, and the specificconditions for preparation of laboratory “film grade” resins are shownin Table II. The comparative resins produced have the characteristicsshown in Table II. TABLE I Experimental Conditions for CatalystEvaluations Reagents: Conditions: Catalyst: 10 mg Temp.: 70° C. TEAL:1.0 mmol Time: 1 hour Ext. Donor Propylene: 1.4 L (0.72 kg)Prepolymerization: in situ

[0041] TABLE II Polymers Prepared Using Ti/Mg Z-N Catalyst in thePresence of Various Donors Activity, Xylene Ex # Donor Al/Si H2, mol %g/g/h Flow Melt Sols., % 1 CMDS 40 0.09 36300 3.2 4.3 2 BMDS 20 0.0525000 2.8 3.7 3 E 40 0.045 30500 2.1 3.8

[0042] In general, the activities of Catalyst X used in Table I may berelatively similar in the presence of the various donors. As seen above,the activities ranged from 25-36,000 g/g/h, depending on conditionsneeded to obtain “film grade” resins (i.e., MF of about 2-3, XS of about3.5-4.5%). Also, BMDS gave very similar polymer properties as comparedto “E” donor. For example, the polymers prepared with both BMDS and “E”donors possessed somewhat more narrow polydispersities (approximately6-6.5) than those prepared in the presence of CMDS. The polymer meltingpoints and heats of fusion and recrystallization were also very similar,and somewhat lower than those from CMDS, indicating a desirablereduction in polymer crystallinity. From ¹³C NMR analyses of theinsoluble fractions, the polymers prepared with this Ziegler-Nattacatalyst in the presence of BMDS and “E” donors showed reductions inchain stereoregularity. The polymer from BMDS possessed a meso pentadlevel of 93%, and the polymer from “E” donor showed 92.4%. Thepolypropylene from CMDS gave an mmmm level of about 94%. Thus, clearlyBMDS yields a desirable decrease in polymer microtacticity, on the sameorder as does “E” donor. Further details are given in Table III. TABLEIII Microtacticities from ¹³C NMR for Polymers Prepared with Catalyst XUsing Various Donors Example 1 2 3 Donor CMDS BMDS E MF, g/10 min 3.22.8 2.1 XS, % 4.3 3.7 3.8 mmmm 93.9 93.0 92.4 mmmr 2.6 3.1 3.1 rmmr 0.50.3 0.4 mmrr 1.5 2.2 2.3 xmrx 0.9 0.4 0.7 mrmr 0.0 0.0 0.0 rrrr 0.0 0.00.0 rrrm 0.0 0.0 0.0 mrrm 0.6 1.0 1.1 % meso 98.2 97.7 97.4 % racemic1.8 2.3 2.6 % error 1.0 0.5 0.8 Defects/1000 C 9.0 11.4 13.2

[0043] Dynamic mechanical analyses were conducted with a RheometricsDynamic Analyzer RDA II to measure the polymer stiffness as a functionof temperature. The results are presented in FIG. 1. The FIG. 1 DMAresults show that resins prepared with BMDS and “E” donors behavesimilarly in terms of stiffness vs. temperature. The desirably reducedstiffness (relative to polymer prepared with CMDS) is due to the reducedpolymer stereoregularity. These results are confirmed by the measured T*values, which show 159.7° C. for polypropylene prepared with BMDS and161.2° C. for “E” donor. The standard donor, CMDS, gives a T* value of164° C. The T* value is the temperature at which the complex modulus ofthe material reaches a discrete value, e.g. 1.4×10⁸ dyn/cm². Some in theindustry have found that a lower T* value, within a specified range,will suggest a wider temperature window in the tenter apparatus, therebymaking it easier to run the film lines at the desired throughput. The T*value is somewhat analogous to the melting point of the polymer, and isa parameter used by some corporations because it can be determined inthe laboratory. Again, it is seen that polymer prepared in the presenceof BMDS compares very favorably against “E” donor. In general, it isdesired to lower the T* value, as long as it is not too low, i.e. itshould not be so low as to adversely affect other parameters. As istypical in polypropylene film processing, a balance between desirablevalues in a number of parameters must be achieved. Relatively low MF andcontrolled XS values are desired for polypropylene film.

[0044] The catalyst system of this invention yields resins havingcontrolled lower polymer stereoregularity, and concomitantly lower T*values, which provide improved film processability.

[0045] Film Properties

[0046] Due to the desirable microtacticity reduction for polymersprepared in the presence of BMDS and E donor, it was of interest tofurther characterize the resins in terms of film behavior. A compressionmolding procedure for the preparation of films that could be studied ina Bruckner film stretcher was developed. Briefly, a Carver CompressionMolding Press was utilized to prepare films, with the methodology beingdeveloped using a standard film grade resin, Resin A (MF of 2.8, XS of3.8-4%). Several parameters (e.g., temperature, pressure, hold times)were varied during the course of the study. Ultimately, a moldingtemperature of 220° C., a low pressure (2000 psi) hold time of 5 min,and a high pressure (36 T (490 MPa) hold time of 5 min were the settledupon conditions. A 15 mil (0.015 in) (0.38 mm) picture-frame mold with 6in.×6 in. (15×15 cm) internal diameter was used to make the films.Before molding, the polypropylene fluffs were stabilized with aphenolic/phosphite antioxidant system, e.g., about 1500 ppmIrganox-1010/Irgafos-168 (2:1) to prevent significant degradation. Foreach of the three samples tested (i.e., Resin A, BMDS, E donor), a totalof about 12 films were compression molded to allow for Bruckner studiesat different temperatures. The thicknesses of the pressed films weregenerally in the 16-17 mil (0.41-0.43 mm) range, with quite acceptablegauge uniformity.

[0047] As noted, the ultimate goal of the work was to evaluate theprocessability of the polymers via the Bruckner film stretcher. Thecompression-molded films were all aged for at least one week prior torunning (to remove any residual stresses in the films that, in anyevent, should be minor due to the fact that compression molding wasused). A few initial Resin A films were stretched at 140° C. undervarious stretch velocity (3-10 Om/min) and stretch ratio (4-7X)conditions to assess behavior. Ultimately, a stretch velocity of 3 m/minand a biaxial stretch ratio of 4 were chosen. The compression-moldedfilms stretched relatively well under these conditions, showing theleast tendency for breaks to occur.

[0048] In terms of the Bruckner studies, temperatures were varied from130 to 150° C. to determine biaxial film stretching behavior and yieldstresses. Generally at each temperature several films were stretched,and an average yield stress was calculated. In most cases, the datareproducibility was quite reasonable. Table IV shows the average yieldstresses of the various polymers as a function of temperature, and theresults are presented graphically in FIG. 2. TABLE IV Bruckner Resultsfor Compression Molded Film Grade Resins Stretching Temperature, Avg.Yield Ex. Resin ° C. Stress, MPa Comments 4 Resin A 135 5.0 Occasionalsmall hole and/or small break 5 ″ 140 4.6 No Breaks 6 ″ 145 2.4 ″ 7 ″150 1.5 ″ 8 BMDS 130 5.0 No Breaks 9 ″ 135 4.8 ″ 10 ″ 140 3.6 ″ 11 ″ 1451.7 ″ 12 ″ 150 1.1 ″ 13 E Donor 130 6.0 Some breaks 14 ″ 135 5.4 NoBreaks 15 ″ 140 3.8 ″ 16 ″ 145 2.4 ″ 17 ″ 150 1.4 ″

[0049] As seen from the above results, the polymer film yield stressesexpectedly decreased in a linear fashion with increasing stretchingtemperature. As is typical, some scatter in the data were evident;however, reasonable correlation coefficients of 0.94-0.98 were obtained.The yield stress results as a fimction of temperature for films preparedwith the fluoroalkylsilane donor (E donor) were relatively similar tothose of the current film grade resin, Resin A. Both resins showed goodfilm stretching behavior and similar dependence of yield stress ontemperature. It was generally expected that the reduced microtacticity(mmmm −92.4%) of the polymer made with E donor would be reflected inperhaps lower (or flatter) yield stresses vs. temperature. The resinprepared with Catalyst X in the presence of BMDS showed desirably loweryield stresses at a given temperature than did the other tested filmresins. Such results were expected based on the reduced microtacticityof the polymer (mmmm −93%). Thus, the results of the Bruckner filmstretching study showed some differences in film properties amongst thevarious resins, with BMDS giving a T350 temperature (i.e., temperatureat which the yield stress is 350 psi [2.41 MPa]) of 144° C., and E donorand Resin A giving 146-146.5° C. From a practical point of view, theresin with the lower processability temperature (i.e., BMDS) may offersome advantage in the tenter portion (due to reduced heating needs, etc.which may translate into increased speeds).

[0050] Selected films that were biaxially oriented (stretch ratio—4X) onthe Bruckner Film 10 Stretcher were cut into about 10 in. strips (about25.4 cm) for obtaining tensile properties. Films of each resin stretchedat 140 and 145° C. were selected for these analyses. Table V gives thetensile properties as measured via an Instron tester. TABLE V TensileProperties for Film Grade Resins Stretched via the Bruckner [Ratio-4X]Ex. 18 19 20 21 22 23 Resin A Resin A BMDS BMDS E Donor E Donor Stretch140 145 140 145 140 145 Temp., ° C. Tens. Str., 4150 3800 3990 3820 46204600 Yld. psi (28.6) (26.2) (27.5) (26.3) (31.8) (31.7) (MPa) Tens.Str., 22200 19600 18800 16500 22900 21600 Brk. psi  (153)  (135)  (130) (114)  (158)  (149) (MPa) Elong., 2.0 2.0 2.2 2.4 2.2 2.2 Yld., %Elong., 81 64 81 80 69 61 Brk., %

[0051] From the above results, it is seen that the oriented films (4X)exhibited tensile strengths at yield in the 3800-4600 psi range(26.2-31.7 MPa), with the films prepared with E donor showing thehighest values and BMDS the lowest. Similar results were noted for thetensile strengths at break, with values typically in the 16-23,000 psi(110-159 MPa) range. All of the films (whether stretched at 140 or 145°C.) gave similar elongations at yield of 2% and break at 50-80%.

[0052] In the foregoing specification, the invention has been describedwith reference to specific embodiments thereof, and has beendemonstrated as effective in providing a method for making aZiegler-Natta catalyst system for the polymerization andcopolymerization of propylene monomer. However, it will be evident thatvarious modifications and changes can be made thereto without departingfrom the broader spirit or scope of the invention as set forth in theappended claims. Accordingly, the specification is to be regarded in anillustrative rather than a restrictive sense. For example, specificcombinations or amounts of co-catalysts, internal donors, and externaldonors, and other components falling within the claimed parameters, butnot specifically identified or tried in a particular catalyst system,are anticipated and expected to be within the scope of this invention.Further, the method of the invention is expected to work at otherconditions, particularly temperature, pressure and concentrationconditions, than those exemplified herein.

We claim:
 1. A catalyst system for the polymerization orcopolymerization of olefins comprising: (a) a Ziegler-Natta catalyst;(b) an organoaluminum compound co-catalyst; and (c) at least oneexternal electron donor comprising n-butylmethyldimethoxysilane (BMDS).2. The catalyst of claim 1 where the Ziegler-Natta catalyst comprises atransition metal compound of the formula MR⁺ _(x) where M is selectedfrom the group consisting of titanium, chromium, and vanadium, R isselected from the group consisting of halogen or a hydrocarboxyl, and xis the valence of M.
 3. The catalyst of claim 1 where in (b) theorganoaluminum compound is triethyl aluminum (TEAL).
 4. The catalyst ofclaim 1 where the Al/Si molar ratio (organoaluminum compound to silanedonor) ranges from about 0.5 to about
 500. 5. A catalyst system for thepolymerization or copolymerization of olefins comprising: (a) aZiegler-Natta catalyst, where the Ziegler-Natta catalyst comprises atransition metal compound of the formula MR⁺ _(x) where M is selectedfrom the group consisting of titanium, chromium, and vanadium, R isselected from the group consisting of halogen or a hydrocarboxyl, and xis the valence of M; (b) an organoaluminum compound co-catalyst; and (c)at least one external electron donor comprisingn-butylmethyldimethoxysilane (BMDS) where the Al/Si molar ratio(organoaluminum compound to silane donor) ranges from about 0.5 to about500.
 6. The catalyst of claim 5 where in (b) the organoaluminum compoundis triethyl aluminum (TEAL).
 7. A polypropylene comprising a propylenepolymer or copolymer having a melt flow of at least about 0.5 g/10 min.and xylene solubles of not more than about 6%, and a meso pentad levelof between about 91 and about 98%.
 8. The polypropylene of claim 7further having a polydispersity ranging from about 4 to about
 10. 9. Thepolypropylene of claim 7 where the polypropylene is formed by a processcomprising: (a) providing a Ziegler-Natta catalyst; (b) contacting thecatalyst with an organoaluminum compound; (c) contacting the catalystwith at least one electron donor comprising n-butylmethyldimethoxysilane(BMDS) simultaneously with or subsequent to (b); (d) introducing thecatalyst into a polymerization reaction zone containing theorganoaluminum compound, the electron donor and propylene monomer; and(e) removing polypropylene homopolymer or copolymer from thepolymerization reaction zone.
 10. The polypropylene of claim 7 where thepolypropylene has a lower processability temperature as compared to apolypropylene formed in the absence of BMDS that is otherwise identical.11. An article formed from polypropylene comprising a propylene polymeror copolymer having a melt flow of at least about 0.5 g/10 min. andxylene solubles of not more than about 6%, and a meso pentad level ofbetween about 91 and about 98%.
 12. The article of claim 11 where thepolypropylene is formed by a process comprising: (a) providing aZiegler-Natta catalyst; (b) contacting the catalyst with anorganoaluminum compound; (c) contacting the catalyst with at least oneelectron donor comprising n-butylmethyldimethoxysilane (BMDS)simultaneously with or subsequent to (b); (d) introducing the catalystinto a polymerization reaction zone containing the organoaluminumcompound, the electron donor and propylene monomer; and (e) removingpolypropylene homopolymer or copolymer from the polymerization reactionzone.